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• yield stress  (22)
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• 1
Electronic Resource
Springer
Rheologica acta 24 (1985), S. 75-91
ISSN: 1435-1528
Keywords: Suspension rheology ; non-Newtonian viscosity ; yield stress ; coal slurry
Source: Springer Online Journal Archives 1860-2000
Topics: Chemistry and Pharmacology , Physics
Notes: Abstract Illinois coal was ground and wet-sieved to prepare three powder stocks whose particle-size distributions were characterized. Three suspending fluids were used (glycerin, bromonaphthalene, Aroclor), with viscositiesη s that differed by a factor of 100 and with very different chemistries, but whose densities matched that of the coal. Suspensions were prepared under vacuum, with coal volume fractionsφ that ranged up to 0.46. Viscosities were measured in a cone-and-plate over a shear rate $$(\dot \gamma )$$ range 10−3−102 s−1. Reduced viscosityη r = η/ηs is correlated in the high-shear limit (η ∞) withφ/φ M∞, whereφ M∞ is the maximum packing fraction for the high-shear microstructure, to reveal the roles of size distribution and suspending fluid character. A new model that invokes the stress-dependence ofφ M is found to correlateη r well under non-Newtonian conditions with simultaneous prediction of yield stress at sufficiently highφ; a critical result is that stress and not $$\dot \gamma$$ governs the microstructure and rheology. Numerous experimental anomalies provide insight into suspension behavior.
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• 2
Electronic Resource
Springer
Rheologica acta 26 (1987), S. 479-484
ISSN: 1435-1528
Keywords: Stress amplification ; plastic flow ; sclera ; myopia ; yield stress ; glaucoma
Source: Springer Online Journal Archives 1860-2000
Topics: Chemistry and Pharmacology , Physics
Notes: Abstract Amplification of effective stress and plastic strain rates after yielding are derived for the anisotropic stress fields of the prolate and oblate spheroidal shells as models of the myopic and hyperopic eyes. Dimensionless closed-form results are presented for arbitrary axis ratio with both constant shell thickness and variable shell thickness, using the constant scleral mass assumption. The results show that the myopic and hyperopic eyes are susceptible to failure by plastic yielding at the equator and pole, respectively, for high intraocular pressures. Experimental data from the equatorial zone of rabbit sclera shows scleral yielding and plastic flow for intraocular pressures greater than 32 mm Hg. Practical applications include glaucoma and pathological myopia.
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• 3
Electronic Resource
Springer
Rheologica acta 34 (1995), S. 406-414
ISSN: 1435-1528
Keywords: Peristaltic ; Casson fluid ; yield stress
Source: Springer Online Journal Archives 1860-2000
Topics: Chemistry and Pharmacology , Physics
Notes: Abstract The problem of blood flow induced by peristaltic waves in a uniform small diameter tube has been investigated. Blood has been represented by a two-fluid model consisting of a core region of suspension of all the erythrocytes, assumed to be a Casson fluid, and a peripheral layer of plasma as a Newtonian fluid. The expressions for dimensionless pressure drop and friction force have been obtained. The results obtained in the analysis have been evaluated numerically and discussed briefly. The significance of the present model over the existing models has been pointed out by comparing the results with other theories both analytically and numerically.
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• 4
Electronic Resource
Springer
Rheologica acta 35 (1996), S. 446-457
ISSN: 1435-1528
Keywords: Pigmented wax ; viscosity functions ; yield stress ; Casson model ; Herschel-Bulkley model
Source: Springer Online Journal Archives 1860-2000
Topics: Chemistry and Pharmacology , Physics
Notes: Abstract The temperature dependent rheological behaviour of a pigment filled wax system is investigated in a cone-and-plate viscometer over a range of shear rates from 60 to 10 000 s−1. A strong influence of water adsorbed by the pigment on rheological properties of the filled system can be found. The increase of the yield stress and the viscosity at low shear rates can be related to a build-up of pigment structures due to growing water content. The flow behaviour can be described by the Casson equation as well as by the Herschel-Bulkley equation. Both formulations are compared and discussed. The Casson model is evaluated more closely by the calculation of characteristic structural parameters of the suspension which are critically discussed.
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• 5
Electronic Resource
Springer
Rheologica acta 36 (1997), S. 498-504
ISSN: 1435-1528
Keywords: Key words Smectic ; modulus ; strain amplitude ; yield stress ; yield strain
Source: Springer Online Journal Archives 1860-2000
Topics: Chemistry and Pharmacology , Physics
Notes: Abstract We have studied the oscillatory shear response of three thermotropic smectic-A liquid crystalline materials with no external aligning field (other than the necessary presence of rheometer plates). Two are polymers (one main-chain and one side-chain) and the other is a small molecule smectic. All three exhibit the classical linear response to oscillatory shear characteristic of a viscoelastic solid at sufficiently small strain amplitudes and frequencies. However, for strain amplitudes exceeding a small critical value, these materials exhibit a strongly nonlinear response to strain, which is characterized in detail. While the low-strain moduli and the critical strain of the three smectics are considerably different, the nonlinear response has some universal character which is presumably related to the low energy for the formation of defects in smectic liquid crystals.
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• 6
Electronic Resource
Springer
Rheologica acta 36 (1997), S. 498-504
ISSN: 1435-1528
Keywords: Smectic ; modulus ; strain amplitude ; yield stress ; yield strain
Source: Springer Online Journal Archives 1860-2000
Topics: Chemistry and Pharmacology , Physics
Notes: Abstract We have studied the oscillatory shear response of three thermotropic smectic-A liquid crystalline materials with no external aligning field (other than the necessary presence of rheometer plates). Two are polymers (one main-chain and one side-chain) and the other is a small molecule smectic. All three exhibit the classical linear response to oscillatory shear characteristic of a viscoelastic solid at sufficiently small strain amplitudes and frequencies. However, for strain amplitudes exceeding a small critical value, these materials exhibit a strongly nonlinear response to strain, which is characterized in detail. While the low-strain moduli and the critical strain of the three smectics are considerably different, the nonlinear response has some universal character which is presumably related to the low energy for the formation of defects in smectic liquid crystals.
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• 7
Electronic Resource
Springer
Rheologica acta 20 (1981), S. 390-402
ISSN: 1435-1528
Keywords: Fiber suspension ; first normal-stress difference ; yield stress ; Casson plot ; cone-plate rheometer
Source: Springer Online Journal Archives 1860-2000
Topics: Chemistry and Pharmacology , Physics
Description / Table of Contents: Zusammenfassung Das Fließverhalten von Vinylonfasern in Siliconöl wird unter stationären Scherbedingungen in einem Kegel-Platte-Rheometer untersucht. Drei Arten von Vinylonfasern einheitlicher Länge, die eine größere Flexibilität als Glasfasern besitzen, werden verwendet, wobei der Fasergehalt zwischen null und 7 Gew.-% variiert. Scherviskosität, erste Normalspannungsdifferenz, Fließspannung und relative Viskosität werden diskutiert. Scher- und relative Viskosität wachsen mit der Konzentration und dem Längen-zu-Durchmesser-Verhältnis und hängen von der Schergeschwindigkeit ab. Die Anwendbarkeit der Ziegelschen Gleichung für die Viskosität von Fasersuspensionen wird geprüft. Die erste Normalspannungsdifferenz wächst mit der Faserkonzentration, dem Längen-zu-Durchmesser-Verhältnis und der Schergeschwindigkeit, und ihr relativer Anstieg ist viel größer als bei Schubspannung und Viskosität und hängt von der charakteristischen Zeit $$\bar \tau$$ ab. Die Fließspannung kann bei großen Längen-zu-Durchmesser-Verhältnissen mittels des Casson-Diagramms schon bei Konzentrationen bestimmt werden, die viel geringer sind als bei Kugelund Pulversuspensionen. Der Einfluß der Faserflexibilität auf die rheologischen Eigenschaften ist nicht vernachlässigbar.
Notes: Summary The steady shear flow properties of suspensions of vinylon fibers in silicone oil were measured by means of a cone-plate type rheometer. Three kinds of vinylon fibers used had no distributions of length and were more flexible than glass fibers and the like. The content of the fibers ranged from 0 to 7 wt.%. Shear viscosity, the first normal-stress difference, yield stress, and relative viscosity were discussed. Shear viscosity and relative viscosity increased with the fiber concentration and the aspect ratio, and depended upon the shear rate. The applicability of Ziegel's equation of viscosity for fiber suspensions was investigated. The first normal-stress difference increased with the fiber concentration, aspect ratio, and shear rate and its relative increase was much larger than for shear stress and viscosity depending on the properties of the characteristic time, $$\bar \tau$$ The yield stress could be determined by Casson plots for large aspect ratio fiber suspensions even in low concentration comparing with the suspensions of spherical particles or powder. The influence of the flexibility of the fibers for the rheological properties of the fiber suspensions can not be ignored.
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• 8
Electronic Resource
Springer
Rheologica acta 20 (1981), S. 403-415
ISSN: 1435-1528
Keywords: Polymer solution ; fiber suspension ; first normalstress difference ; yield stress ; light permeation
Source: Springer Online Journal Archives 1860-2000
Topics: Chemistry and Pharmacology , Physics
Description / Table of Contents: Zusammenfassung Die rheologischen Eigenschaften von Vinylon-Faser-Suspensionen in Polymerlösungen bei stationären Scherströmungen werden untersucht. Dabei werden Scherviskosität, erste Normalspannungsdifferenz, Fließspannung, relative Viskosität und noch andere Eigenschaften diskutiert. Drei Arten von flexiblen Vinylonfasern von jeweils einheitlicher Länge und drei Arten von Polymerlösungen, die bemerkenswerte Abweichungen vom newtonschen Verhalten zeigen, werden dafür verwendet. Scherviskosität und relative Viskosität (η r ) wachsen mit dem Fasergehalt und dem Längen-zu-Durchmesser-Verhältnis an und sind von der Schergeschwindigkeit abhängig, jedoch merklich nur im Bereich kleiner Schergeschwindigkeiten. Dieses Verhalten kontrastiert mit demjenigen von Vinylon-Faser-Suspensionen in newtonschen Flüssigkeiten. Die erste Normalspannungsdifferenz wächst mit zunehmender Faserkonzentration zuerst geringfügig, um danach abzunehmen und bei hohen Konzentrationen sogar niedrigere Werte anzunehmen als das reine Suspensionsmittel. Mit Hilfe einer modifizierten Casson-Gleichung kann eine Fließspannung bestimmt werden. In den untersuchten Systemen hängen die Fließeigenschaften auch von der Viskosität des Suspensionsmittels ab.
Notes: Summary The rheological properties of vinylon fiber suspensions in polymer solutions were studied in steady shear flow. Shear viscosity, first normal-stress difference, yield stress, relative viscosity, and other properties were discussed. Three kinds of flexible vinylon fibers of uniform length and three kinds of polymer solutions as mediums which exhibited remarkable non-Newtonian behaviors were employed. The shear viscosity and relative viscosity (η r ) increased with the fiber content and the aspect ratio, and depended upon the shear rate. Shear rate dependence ofη r was found only in the low shear rate region. This result was different from that of vinylon fiber suspensions in Newtonian fluids. The first normal-stress difference increased at first slightly with increasing fiber content but rather decreased and showed lower values for high content suspensions than that of the medium. A yield stress could be determined by using a modified equation of Casson type. The flow properties of the fiber suspensions depended on the viscosity of the medium in the suspensions under consideration.
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• 9
Electronic Resource
Springer
Rheologica acta 27 (1988), S. 263-272
ISSN: 1435-1528
Keywords: Steady shearviscosity ; suspension ; filledpolymer melt ; yield stress
Source: Springer Online Journal Archives 1860-2000
Topics: Chemistry and Pharmacology , Physics
Notes: Abstract Filled polymeric liquids often exhibit apparent yielding and shear thinning in steady shear flow. Yielding results from non-hydrodynamic particle—particle interactions, while shear thinning results from the non-Newtonian behavior of the polymer melt. A simple equation, based on the linear superposition of two relaxation mechanisms, is proposed to describe the viscosity of filled polymer melts over a wide range of shear rates and filler volume fraction. The viscosity is written as the sum of two generalized Newtonian liquid models. The resulting equation can describe a wide range of shear-thinning viscosity curves, and a hierarchy of equations is obtained by simplifying the general case. Some of the parameters in the equation can be related to the properties of the unfilled liquid and the solid volume fraction. One adjustable parameter, a yield stress, is necessary to describe the viscosity at low rates where non-hydrodynamic particle—particle interaction dominate. At high shear rates, where particle—particle interactions are dominated by interparticle hydrodynamics, no adjustable parameters are necessary. A single equation describes both the high and low shear rate regimes. Predictions of the equation closely fit published viscosity data of filled polymer melts.
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• 10
Electronic Resource
Springer
Rheologica acta 32 (1993), S. 380-397
ISSN: 1435-1528
Keywords: Apparent viscosity ; shear viscosity ; shift factor ; model fluids ; yield stress
Source: Springer Online Journal Archives 1860-2000
Topics: Chemistry and Pharmacology , Physics
Notes: Abstract Considering a number of model fluids, the relation between the (measurable) apparent viscosity η a and the (true) shear viscosity η is studied for some commonly used viscometers, like capillary, slit, plate-plate and concentric cylinders (including the influence of the bottom of the cylinder), as well as for one laboratory type of viscometer. As long as η is a purely monotonic function, a shift factor β 〈 1 allows one to deduce η from η a . Though in general variable, it frequently suffices for practical purposes to use a constant shift factor (the constant being characteristic of the type of viscometer used). This does not apply to dilute solutions or any fluids with two plateau values for η. For plastic fluids, it is shown that Casson or Bingham behavior can — if valid at all — only describe the high shear stress limit of η a .
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